An Orthogonal Frequency Division Multiple Access (OFDMA) access is adopted in a Long Term Evolution (LTE)-Advanced (LTE-Advanced) system, and information of users in a cell is borne over different sub-carriers which are orthogonal to each other to thereby avoid multi-access interference between users. However it is not possible for the OFDMA access per se to inhibit inter-cell interference, and therefore interference influencing the performance of the LTE-Advanced system originates primarily from an adjacent cell, i.e., inter-cell interference. Further, networking at the same frequency or with a frequency multiplexing factor close to 1 is preferentially adopted in network deployment for the LTE-Advanced system in order to achieve higher spectrum utilization. In a traditional cellular system with networking at the same frequency, a user at the edge of a cell is subject to interference of a signal from an adjacent cell, and such interference may seriously limit the Quality of Service and the throughput of the user at the edge.
As illustrated in FIG. 1, a base station serving a user 1 is a base station 1, a base station serving a user 2 is a base station 2, and a base station serving a user 3 is a base station 3, for example. In the downlink, the user 1 receives interference signals of the base stations 2 and 3 in addition to a wanted signal of the base station 1; and alike, the user 2 receives interference signals of the base stations 1 and 3 in addition to a wanted signal of the base station 2, and the user 3 receives interference signals of the base stations 1 and 2 in addition to a wanted signal of the base station 3. In another word, the signals emitted from the base stations 1, 2 and 3 can be received by all the users 1, 2 and 3.
Therefore if a coordinated group is composed of the base stations 1, 2 and 3, then Coordinated Multi-Point (CoMP) transmission technology is used to transmit collectively the signals to the users 1, 2 and 3 and to pre-inhibit the interference signals at transmitters to thereby effectively reduce inter-cell interference and improve the signal to interference and noise ratios of signal reception of the respective users, thus improving the Quality of Service and the throughput of a user at the edge of a cell and improving the spectrum utilization of the system. CoMP transmission is implemented under the principle as illustrated in FIG. 2, where a coordinated group is composed of a plurality of base stations (or a plurality of radio frequency nodes spaced far away) and a set of coordinated cells is composed of serving cells of all the base stations in the group so that respective users in the coordinated cells are paired and have resources allocated thereto uniformly and according to the result of paring the users, the respective coordinated base stations jointly process and then collectively transmit downlink signals of the paired users. For example, coordinated groups are composed respectively of base stations 1, 2 and 3 or base stations 4, 5 and 6, and devices in the respective coordinated groups responsible for joint processing of signals can be arranged on one of the base stations or on a separate network entity connected with the respective base stations and a core network.
As illustrated in FIG. 3, in an LTE system, downlink transmission control of the uncoordinated system is performed separately at respective base stations, and for a plurality of users in serving cells, for which downlink data is to be transmitted concurrently, base stations schedule the users periodically according to feedback information of the users on the reception qualities of downlink channels, where a scheduling process includes the following operations:
S301. Serving base stations of users transmit downlink common pilots, and the users estimate channel H matrixes of respective downlink channels according to the common pilot signals of the downlink channels;
The common pilots are sequences known to the users, and the users can estimate the channel H matrixes of the downlink channels according to received signals and the known common pilot sequences.
S302. The users estimate the post-detection signal to interference and noise ratios SINR2 of the corresponding downlink channels according to the estimated channel H matrixes of downlink data and a reception and detection scheme and search with the SINR2 for Modulation and Coding Scheme (MCS) levels of the downlink channels;
Particularly there are 15 combinations of modulation and coding schemes in total, and with the identifiers of the 15 MCS levels, each MCS level corresponds to an SINR in a specific range. Therefore the corresponding MCS level can be found with the post-detection signal to interference and noise ratio SINR2.
S303. The users feed the found MCS levels corresponding to the downlink channels back to the serving base stations;
S304. The serving base stations of the users determine a downlink radio resource allocation scheme according to the MCS levels corresponding to the downlink channels fed back from all the users of the base stations;
Specifically, a frequency resource for each downlink channel is a frequency resource block, and for feeding back the MCS level of the downlink channel, the user can feed back the serial number of each frequency resource block and the corresponding MCS level or a preset number of preferred frequency resource blocks with the highest MCS levels to the serving base station, and the base station can select several scheduled users with the highest MCS levels corresponding to the respective frequency resource blocks according to a preset criterion and the feedback information of the users and transmit preferentially the downlink data of the scheduled users.
S305. The base stations adjust the MCS levels fed back from the scheduled users and also notify the scheduled users about the adjusted MCS levels.
S306. The base stations modulate and encode the downlink channels of the users according to the adjusted MCS levels and transmit the downlink data; and
S307. The scheduled users detect the downlink channels and receive the downlink data.
The inventors of the present application have identified that in downlink transmission of the coordinated multi-point transmission system, the signal of a user has to be weight-preprocessed at the transmitter but the weight of the preprocessing is unknown to the user side, therefore the post-detection signal to interference and noise ratio SINR2 can not be estimated accurately merely from the downlink channel H matrix estimated from the common pilot and the reception and detection scheme, and consequently the MCS levels corresponding to the respective frequency resource blocks can not be determined accurately and the efficiency of downlink transmission control can not be guaranteed. Therefore the existing uncoordinated downlink transmission control solution can not be applied directly to the coordinated multi-point transmission system.